Hugh Thomson has a fascinating article on early agriculture in Bronze Age Britain. The record shows that much of "England" was cleared by 1000 BC (I'm not certain if this includes Wales, Scotland or Ireland . . . and my guess would be no as those areas possess slightly more rugged terrain). Those ancient forests that pre-date the dawn of time just aren't accurate. It takes only a few centuries for a forest to regrow, weeding out smaller trees and become "old growth"; it would seem that Enland's old growth came after the Romans.
The myth panders to our need for a sense of loss. There is an
undercurrent of regret running through our history. A nostalgia for what
could have been: the unicorn disappearing into the trees; the loss of
Roman Britain; the loss of Albion; the loss of Empire. We are forever
constructing prelapsarian narratives in which a golden sunlit time — the
Pax Romana, the Elizabethan golden age, that Edwardian summer before
the First World War, a brief moment in the mid-1960s with the Beatles —
prefigure anarchy and decay. Or the cutting down of the forest.
Read the whole article.